1. Field of the Invention
The present invention relates to an apparatus capable of verifying an electronic document and a corresponding method.
2. Description of the Related Art
Signature technology employing an electronic signature and a time stamp signature is known. Also, there is known a technique for, when an electronic document added with an electronic signature is subsequently edited (i.e., updated after the addition of the first electronic signature), adding another electronic signature corresponding to the subsequent editing (Japanese Patent Laid-Open No. 2003-216601). As described in Patent Document 1, a reason why the second electronic signature is added corresponding to the subsequent editing resides in explicitly indicating that a person who has edited the electronic document is responsible for the details of the editing made by the person.
Herein, a time stamp signature and an electronic signature associated with an electronic document are collectively referred to as a signature. Accordingly, the expression “adding an electronic signature” is used to describe any of the following cases: (1) a case of adding an electronic signature, (2) a case of adding a time stamp signature, and (3) a case of adding both an electronic signature and a time stamp signature.
Further, Japanese Patent Laid-Open No. 2003-216601 discloses a method of, in a process of verifying an electronic document subjected to editing or added with a signature corresponding to the editing, extracting an editing assurance signature from edit records before one generation, and verifying the extracted signature. More specifically, the disclosed method performs the verification by comparing a hash value calculated from editing differential information before one generation with a hash value decoded from a history assurance signature by using a public key for a person who made the editing in the same generation. When the verification of the history assurance signature is completed, update differential information before one generation is acquired and the electronic document is restored based on the text of the electronic document and the update differential information before one generation.
With the technique disclosed in Japanese Patent Laid-Open No. 2003-216601, however, when an electronic document is added with a plurality of signatures and those signatures are to be verified together, a long verification time may be required. Accordingly, there is a demand for verifying an electronic document added with a plurality of signatures in a shorter verification time.